A known sensor of this type, disclosed in U.S. Pat. No. 4,685,464, is configured in the form of a clothes line clip. The end of a patient's finger can be inserted between the two jaws of the clip. A hollow housing is placed on each of the two jaws. The housings face and are turned toward each other to form a trough to receive the end of the finger. An indentation, in the bottom of this trough, receives the transmitter or, respectively, the receiver. A thin layer of a light permeable material closes the indentation from the outside and covers the transmitter or, respectively, the receiver, forming a light permeable window. The two hollow housings are made of a soft elastic material, for example, silicon rubber, forming pads to distribute the pressure of the device uniformly on the surface of the finger.